1. Field of the Invention
This invention relates to synchronizing signal decoding. More particularly, the invention relates to methods and apparatus for decoding a digital synchronizing signal.
2. Description of the Prior Art
We have recently proposed (UK patent application no. 8604687 filed Feb. 26, 1986) a modification to the AES/EBU (Audio Engineering Society/European Broadcasing Union) format for serial transmission of audio data, in which modification of the channel status information words are in socalled free-format, that is, the number of bits in each channel status information word may vary within limits from one channel status information word to the next. The above-mentioned proposal may be likewise applied to the words comprising the user bits, but for simplicity we will, throughout this specification, refer mainly to the channel status information words. The proposal gives rise to a particular problem at the decoder, because although each of the synchronizing signals forming part of the channel status information words consists of a fixed number of bits in a predetermined pattern, the synchronizing signals do not necessarily occur at fixed intervals.
To maintain low redundancy, the modified format uses synchronizing signals consisting of sixteen bits, and these are associated with the remainder of the channel status information word, which at a minimum may consist of a further 40 bits. The probability of the 16-bit synchronizing signal pattern occurring in a 40-bit block is 1 in 25/2.sup.16 ; which represents about 0.04% chance. This is unacceptable, and the known technique of locking to the synchronizing signals only when two successive synchronizing signals have been successfully decoded is therefore used. However, this known technique has hitherto only been applied where the synchronizing signals occur at fixed intervals.